Electrophotography is the most important non-impact printing technology for today's reprographic industries. The electrophotographic copying or printing process normally creates images on a coated polymeric substrate in five steps, with the individual steps of the process generally include the following: (1) depositing a uniform electric charge onto a photoconductor drum in the dark; (2) creating an electrostatic latent image on the photoconductor by exposing the photoconductor to an oscillating narrow laser beam that is turned on and off digitally; (3) exposing the photoconductor to toner particles, wherein toner particles having the correct polarity adhere to the exposed latent image; (4) passing the media to be printed between the photoconductor and a transfer corona to cause the toner particles to transfer from the photoconductor to the media; and (5) fixing the transferred toner particles to the media by one of various procedures known in the art.
Technological advances in electrophotography in recent years have brought an increase in the popularity of color electrophotographic copiers and printers. Unlike a monochrome copier wherein only one black toner is employed, full color copying generally requires four toners including yellow, magenta, cyan, and black. Since a separate imaging process is required for each of the four toners, color copiers and printers are much slower and more expensive than their monochrome counterparts. The recording media suitable for color copiers or printers must meet more stringent requirements to provide a true full-color reproduction of the original.
One important use of color electrophotographic copiers or printers is to make overhead projection transparencies wherein a transparent receptor film is used as the media to receive the image of the original. There is increasing demand for high performance transparent receptor film for color overhead projection transparency uses. Current commercial receptor media consist of a polymeric substrate such as polyethylene terephthalate (PET) and one or more thin layers of organic coatings coated thereon for better imaging quality and feeding performance. Uncoated PET films give poor toner adhesion and image quality and unreliable feeding performance.
Current commercial receptor media are frequently deficient in color fidelity, color density, toner adhesion, and scratch resistance. Unreliable transport of the media through the copier or printer due to inappropriate surface properties is also a common problem. Feedability is the most important design parameter since if the imaging media does not feed through a copier or printer none of the media's other qualities is relevant.
Good toner adhesion is also very important. If the toner does not adhere well to the receptor layer, incomplete toner transfer from the photoconductor to the receptor layer can occur. This can result in hollow characters and poor image resolution. Poor toner adhesion can also result in images being abraded off during handling.
Since transparencies are used for overhead projection, it is essential to design a receptor layer that gives high image quality and a true projection of the original. Poor color fidelity is often related to improper fusing of the toner particles in the toner-receptive coating.
Good thermal and mechanical stabilities are also necessary in order to avoid scratches, buckling, and loss of planarity during or after the converting, copying, and handling processes.
Although various recording media have been proposed for color electrophotographic copying or printing applications, none of them has satisfied the substantial need in the art.
U.S. Pat. No. 3,854,942 discloses a transparency for use in a multi-colored xerographic reproduction process comprising a transparent, thermoplastic film sheet having at least one surface coated with a mixture consisting of a vinyl chloride-acetate copolymer resin and an acrylic resin in a weight ratio of between about 6:4 and 7:3, with a wetting agent in said mixture in an amount between about 2.5 to 25% by weight of said mixture. A percentage of a particulate material is also incorporated in the coating to reduce static charge on the transparency and permit easier handling thereof.
U.S. Pat. No. 5,229,188 discloses a transparent laminate film suitable to receive a color toner image, having disposed thereon at least a first transparent layer containing a heat-resistant transparent resin, and a second transparent layer containing a second transparent resin, wherein the transparent resin of the second transparent resin layer has a compatibility with the binder resin of a toner to be fixed thereon, and a larger storage elasticity modulus than that of the binder resin of the toner at a fixing temperature of the toner.
U.S. Pat. No. 5,208,093 discloses a film used for electrographic printing, wherein the film is coated with a polymeric receptor layer having an equivalent or lower storage elasticity modulus than a toner resin used for forming images on said film.
European Patent Application No. 0 657 782 A1 discloses a toner imageable film comprising a transparent film substrate bearing on one major surface thereof a toner receiving layer, wherein the toner receiving layer has a lower softening point than the toner with which it is used.
International Patent Application WO 96/02023 discloses an image receiving film for electrography which can prevent the occurrence of an oil pooling phenomenon by adding 0.1-100 parts by weight, based on 100 parts by weight of image-formable resin, of a porous silica having a surface area of not less that 350 m.sup.2 /g and an average particle diameter in the range of from 0.05 to 100 micrometers and/or polysiloxane particles.
European Patent Application No. 0 633 508 A2 discloses an image-receiving sheet comprising a substrate sheet, an image receiving layer composed mainly of a polyester resin comprising an acid moiety and a diol moiety of a modified bisphenol A of Formula (I) as disclosed therein, and an opaque porous resin layer as a detection mark that can turn transparent upon heating. The opaque porous resin layer is formed by coating a resin varnish comprising a resin selected from an acrylic resin, a polyester resin, a vinyl chloride/vinyl acetate copolymer resin, and mixtures thereof, a good solvent having a relatively low boiling point and a poor solvent having a relatively high boiling point on said image-receiving sheet and drying the resultant coating.